Hereinafter, a conventional retransmission control method is explained. For example, for error control, Forward Error Correction code (FEC) and Automatic Repeat reQuest (ARQ) are available, and in packet transmission, error free transmission must be guaranteed, so that error control by ARQ is essential. Particularly, in a system that improves the throughput by selecting an optimum modulation method and coding method according to the propagation path status (adapted modulation and demodulation/error correction), packet errors are inevitable, so that an HARQ method with an FEC function is necessary.
As the HARQ method, Type-I HARQ in which a retransmitting packet is identical to an original packet, and Type-II HARQ in which a retransmitting packet is different from an original packet, are available.
Herein, an example of the Type-II HARQ is explained. In Type-II HARQ, data bits are transmitted at the time of initial transmission and parity bits for error correction are transmitted at the time of retransmission in principle, and as an example, a system using turbo codes to which the Type-II HARQ is applied is explained herein (refer to Non-patent Document 1). For example, in a system using turbo codes, a communications device on a transmitting side encodes a data signal sequence at a code rate R, and then thins-out redundant bits (parity bits) after being encoded based on a predetermined method of elimination, and then transmits these. Then, at the time of retransmission, a packet composed of only an additional parity different from the initially transmitted packet is transmitted. On the other hand, in a communications device on a receiving side, the initially transmitted receiving packet stored in a receiving buffer and a retransmitting packet are synthetically encoded, and decoding is carried out at a smaller code rate according to the number of times of retransmission.
In the Type-II HARQ, this series of processes is repeated until no error is detected, whereby error-free transmission is realized, and furthermore, by improvement in coding gain, the receiving performance is improved.
Non-patent document 1
J. Xu, “Turbo Coded Hybrid Type II ARQ System” Master's thesis, Chalmers University of Technology, School of Electrical and Computer Engineering, 2002.
However, in the retransmission control method using turbo codes in the document, the larger the number of bits to be deleted, the longer the distance from the Shannon limit, resulting in deterioration in performance. In addition, in the retransmission control method using turbo codes, even when an additional parity is transmitted at the time of retransmission, it is unknown whether the selected parity is optimum, so that there is a possibility that the original performance of turbo codes cannot be obtained.
The invention was made in view of the circumstances, and an object thereof is to provide a retransmission control method and a communications device which can always obtain the original performance of error correcting codes.